Silicon dendritic web crystals are long, thin ribbons of single crystaline material of high structural quality which are grown in the (111) orientation. The current impetus for developing silicon dendritic web crystals are their application to the production of low cost, highly efficient solar cells for direct conversion of sunlight into electrical energy. The thin ribbon form of the crystal requires little additional processing prior to device fabrication, in contrast to wafer substrates from the more traditional Czochralski crystal which must be sliced, lapped and polished prior to use, a costly process even though large volume economies are practiced. Additionally, the rectangular shape of the silicon ribbon leads to efficient packing of individual cells into large modules and arrays of solar cells. For more information concerning techniques of growing dendritic web crystals, see Seidensticker, "Dendritic Web Growth of Silicon," Crystals 8 p. 146-172 (1982), incorporated by reference herein.
An important aspect of successful dendritic web crystal growth is the configuration of the shields and the lid used to cover the hot silicon melt. In the past, lids having a so-called "dog bone" shaped slots have been used with some success. Such lids also typically include a pair of outboard holes spaced a predetermined distance from the ends of the slot. See U.S. Pat. No. 4,751,059, incorporated by reference herein. These outboard holes assist in maintaining the growing dendritic web crystal at a constant width. More recently, the dog bone shaped slot has been replaced with the so-called "bowtie" shaped slot as disclosed in my co-pending application Ser. No. 07/092,796 now U.S. Pat. No. 4,828,808. Additionally, U.S. Pat. No. 4,786,479 discloses the use of sublids to control heat radiation.
While these lid configurations have proven effective, they have been unable to achieve 100% feed rate capability combined with high productivity. For example, in the bowtie configuration disclosed in my co-pending application Ser. No. 07/092,796now U.S. Pat. No. 4,828,808, a production rate of only 6.2 cm.sup.2 /min. was obtained at 100% feed rate capability.
As used herein the term "100% feed rate capability" means that the molten material (such as silicon) contained by the susceptor is being replaced at the same rate the material is being withdrawn by virtue of the growing dendritic web being pulled from the melt. In the past, replenishment of the melt at 100% feed rate caused "ice" formation in the melt because of the cooler replenishment pellets being dropped therein. This in turn resulted in decreased productivity to the point where only about 75% of feed rate capability could be achieved before nucleation or "ice" formation occurred.
Other configurations have produced up to 7.4 cm.sup.2 /min. production rate, at the expense of achieving only 75% feed rate capability. Accordingly, it would be useful to achieve 100% feed rate capability while simultaneously achieving high production rates of crystal growth.